AppEngine: Step-by-Step Debugging

Question

While working with AppEngine locally (i.e. using dev_appserver.py), is there anyway to do a step-by-step debugging? It is a too old fashion to use logging.info() or similar functions to show the values of all the variables in the code and decide where the error is.

Answer 1

To expand a little bit on codeape's answer's first suggestion: Because dev_appserver.py mucks about with stdin, stdout, and stderr, a little more work is needed to set a "code breakpoint". This does the trick for me:

import sys
for attr in ('stdin', 'stdout', 'stderr'):
    setattr(sys, attr, getattr(sys, '__%s__' % attr))
import pdb
pdb.set_trace()

You'll have to run dev_appserver.py from the command line rather than via the GUI App Engine Launcher. When the pdb.set_trace() line is executed, you will be dropped into the pdb debugger at that point.